Surface Acoustic Wave (SAW) Torque Sensing is an emerging technology for automotive, transportation, rail and other similar industries as used in power train to measure the actual torque on board. Significant research and development efforts enhanced mass-production of SAW torque sensors at a cost-effective price. Certain engine, transmission and driveline can often be controlled more precisely using SAW torque technology. Used in appropriate applications, complex control algorithm and system development, test, evaluation and qualification time can often be significantly reduced with real-time torque sensor measurement that can provide feedback for closed-loop control.
The SAW device consists of two piezoelectric elements which sense the strain in the parent material when the torque is applied. Depending upon the torque applied the parent material surface gets strain in the range of 500 to 2000 microstrain. To measure the accurate strain from the parent material, the sensor button needs to have 100% surface contact with the parent material for effective strain transfer. Apart from this functional requirement, the joining should withstand the automotive harsh and corrosive environment and also the centrifugal force created during the high revolutions per minute (RPM) of the parent part. Adhesives used are failing under dynamic conditions and in salt spray test. Similarly, the assembly cycle time is high increasing the product cost, and the adhesive process is not suitable for high volume production because of the difficulty in automation and time consuming curing process.
Many applications are dependent on dissimilar material joints. Due to the different chemical, mechanical and thermal behaviors of materials, dissimilar materials joining present challenges significantly different than similar materials joining. These methods can work for varying applications. The existing solution uses a custom developed adhesive used mainly for metal to metal attachment for aerospace application such as cynoacrylate based adhesives. The adhesive comes with two solutions and needs to be mixed in a proper ratio as defined by the manufacturer and then applied in drops using a dispensing machine
Such adhesive needs to be cured at high temperature for longer durations, for example the cynoacrylate based adhesive and the assembly needs to be cured in the thermal chamber for about 20 hours at 170 degree C. Before placing it onto the chamber the surfaces of the shaft and button need to be cleaned without any dirt and grease by neutralizer and conditioner. Apart from this the parent material on to which the SAW sensor housing need be mounted such as Flexplate, output shaft, input shaft of transmission system are quite heavy and this needs to be placed in the thermal chamber for curing. This calls for big thermal chambers and storage space resulting high investment. Also the flatness of the parts is very critical to achieve the better contact surface. The other requirement for using dissimilar material for SAW torque application is, to have maximum strain transfer, high yield strength and low hysteresis.
Based on the foregoing it is believed that a need exists for improved joining method by welding of dissimilar materials using laser welding. By using such a methodology, micro-crack free weld joint can provide a robust joint system for the lifetime of any automobile.